This invention relates to the treatment of a hot aqueous brine solution containing various dissolved components, such as iron and silica, to inhibit precipitation therefrom of undesirable scale, such as iron silicate scale. More particularly, the invention relates to such a treatment wherein the scale is formed when the brine is produced and handled in a manner so that its temperature and pressure are reduced, as when a geothermal brine is processed to recover its heat content.
The solubility of most ions in solution decreaees with a decrease in temperature or pressure of the solution. If dissolved ions are present near their saturation concentration in the solution, a slight reduction in the temperature or pressure of the system can result in precipitation of a portion of these ions, which often combine and deposit as a scale on any solid surface with which they come into contact, such as the vessel or conduit in which the solution is confined.
One example of such a solution is a liquid stream containing hot water which is passed through a conduit in an industrial operation under conditions, such as a lowering of the pressure, which flash at least a portion of the hot water to steam. If the hot water is a brine containing appreciable amounts of dissolved salts, this flashing is often accompanied by the formation of scale on the surfaces of the conduit contacted by the fluid stream. Scale deposits tend to build up over a period of time and restrict further fluid flow through the conduit, requiring either operation at a reduced flow rate or an increase in the amount of power used to move the fluid through the conduit. In extreme cases, the conduit can become completely plugged with scale and the industrial operation must be shut down for maintenance.
Industrial operations for generating steam power often are hampered by the buildup of scale deposits caused by flashing of hot water containing dissolved salts. Among the various methods used to produce power from steam are fossil-fuel steam generators, nuclear steam supply systems, and geothermal generator units.
Geothermal steam and hot brines are found in naturally occurring, large subterranean reservoirs in many regions of the world. If located at readily accessible sites, geothermal steam and water or brine have, for some time, been used for therapeutic purposes, for industrial processes, or for direct heating. Although interest in developing geothermal resources further for these purposes still exists, recently the principal effort has been towards developing these partially renewable resources for production of electric power.
Techniques are known whereby hot geothermal fluids can be used to generate electric power. Pressurized geothermal water or brine, having a temperature above about 400.degree. F., can be flashed to a lower pressure and the steam generated by flashing can be used to drive a steam turbine in combination with an electrical generator. However, formidable problems are generally encountered in handling and disposing of large amounts of heavily contaminated and frequently highly saline geothermal liquids. Consequently, production of electricity from geothermal waters on a commercial scale has been difficult and costly to achieve.
One of the most serious problems encountered in using hot geothermal liquids for producing electric power results from scaling of the equipment used to confine and contain the liquid. Because geothermal liquids have usually been confined in subterranean reservoirs for extraordinarily long periods of time at elevated temperatures, large amounts of minerals are leached from the reservoirs into the brine. Typically, salts and oxides of heavy metals, such as lead, zinc, iron, silver, cadmium, and molybdenum, are found in geothermal brine. Other more common minerals, such as calcium and sodium, are also dissolved in the brine, as are naturally occurring gases, including carbon dioxide, hydrogen sulfide, and methane. An especially troublesome component of the hot brine may be silica, which is found in large concentrations in the form of silicic acid oligomers.
Various proposals have been made to decrease the scale formation in equipment used in producing and handling geothermal brine. In "Field Evaluation of Scale Control Methods: Acidification," by J. Z. Grens et al., Lawrence Livermore Laboratory, Geothermal Resources Council, Transactions, Vol. 1, May 1977, there is described an investigation of the scaling of turbine components wherein a geothermal brine at a pressure of 220 to 320 p.s.i.g. and a temperature of 200.degree. to 230.degree. C. (392.degree. to 446.degree. F.) was expanded through nozzles and impinged against static wearblades to a pressure of 1 atmosphere and a temperature of 102.degree. C. (215.degree. F.). In the nozzles, the primary scale was heavy metal sulfides, such as lead sulfide, copper-iron sulfide, zinc sulfide, and cuprous sulfide. Thin basal layers of fine-grained, iron-rich amorphous silica promote adherence of the primary scale to the metal substrate. By contrast, the scale formed on the wearblades comprised cuprous sulfide, native silver and lead sulfide in an iron-rich amorphous silica matrix. When the brine which originally had a pH of 5.4 to 5.8 was acidified with sufficient hydrochloric acid to reduce the pH of the expanded brine to values between 1.5 to 5.0, scaling was eliminated. However, acidification of hot brines to such low pH levels may promote corrosion of the brine-handling equipment to such levels that corrosion defeats the use of acid for scale control.
While the aforementioned treatments have met with some success in particular applications, the need exists for a further improved treating process to reduce scale deposition during the handling of hot aqueous brines, especially geothermal brines.
Accordingly, the present invention provides a method for inhibiting the buildup of scale, especially iron silicate scales, on surfaces of the fluid-handling equipment contacted by hot geothermal fluids. The invention is particularly advantageous for treating a naturally acidic geothermal fluid, containing at least a portion of a geothermal brine, utilized for the generation of electric power so as to inhibit the depostion of metal silicate scale from the geothermal brine onto the fluid-handling equipment.